Wide energy band gap semiconductor materials such as a III-nitride semiconductor and silicon nitride (SiC) have had a wide range of applications to light emitting elements in blue-ultraviolet range and high-frequency high-output devices and have been growing in importance more and more in late years. These materials are very expensive for a bulk substrate, so that heterogeneous materials are used as a substrate in fabricating devices. Desired semiconductor materials are formed as thin film on the abovementioned heterogeneous-material substrate by crystal growth such as chemical vapor deposition (CVD) method or molecular beam epitaxy (MBE) method. As materials for a III-nitride semiconductor represented by gallium nitride (GaN), for example, industrially inexpensive sapphire (Al2O3) is generally used, and silicon (Si) is used in a cubic crystal SiC substrate. However, these substrates are different from semiconductor materials formed thereon in lattice constant. For example, there exist a lattice mismatch of about 12% between a GaN and a sapphire substrate, and a lattice mismatch of as large as about 20% between SiC and a Si substrate. These lattice mismatches cause large strain between a substrate and a grown layer, producing a large number of defects such as threading dislocation and plane defect in the thin film, which markedly deteriorates device operation. For example, a III-nitride semiconductor formed on sapphire and SiC film formed on a SiC substrate have defects on the order of 108 to 109/cm2. These defects lower a lifetime of a light emitting device and produce leak current in an electronic device.
It is effective to form a semiconductor layer acting as a buffer between a substrate and a grown film to suppress the development of defects and thereby to achieve a high quality semiconductor film. This method has been an effective thin-film forming technique related to lattice mismatch. In a device using a III-nitride semiconductor, for instance, a GaN layer grown at lower temperatures is inserted as a buffer between a sapphire substrate and the III-nitride semiconductor layer to upgrade the quality of crystal. In forming a SiC buffer layer on a Si substrate, the Si substrate is heated at temperatures of about 1300° C. using hydrocarbon-based gas such as propane (C3H8) before the SiC layer is crystal-grown. The buffer layer functionally prevents strains from propagating by containing defects therein, suppressing the production of defects in the film formed on the buffer layer. Another method for achieving a high-quality thin film has been to control the direction in which defects propagate by pattering the substrate and grown film. This method is shown in FIG. 3 as a prior art disclosed in Japanese Patent Application Laid-Open Publication No. 12-164929.
On a semiconductor substrate 101 is formed a semiconductor film 102 different in lattice constant therefrom. Then the semiconductor film is patterned by using photo lithography and etching. At this point, facets 103 with given a certain angle to the upper surface of the substrate are formed on the semiconductor film. When a semiconductor film 104 comprised of the same materials as film 102 is grown on the patterned film 102, threading dislocations 105 extending toward the perpendicular direction from the substrate are refracted on the facets 103 to be oriented to the parallel direction with respect to the substrate surface, which suppresses the propagation of the dislocations to the perpendicular direction. These dislocations are again bent toward the perpendicular direction with respect to the semiconductor substrate when grown films extending from two opposing facets meet together. This divides the grown films into a lower defect-density area 106 and a higher defect-density area 107. The fabrication of a device in area 106 enables the formation of a high-performance semiconductor element. Another prior method of directly patterning the substrate has been disclosed in Japanese Patent Application Laid-Open Publication No. 12-178740 as shown in FIG. 4. This is fabricated in such a manner that a plurality of facets 202 arranged in [−110] direction are formed on Si substrate 201 with a (001) surface orientation and then a SiC film 203 is grown throughout substrate 201. Facets 202 will orient plane defects 204 produced in SiC film 203 toward (111) and (−1-11) planes, for example, thereby causing defects extending from the opposing facets to disappear together. This method is effective for a hard grown film difficult to pattern, for example, SiC layer. The above method allows decreasing threading dislocations extending to the surface of the grown film to 107/cm2.
Upgrading performances of devices requires further reduction of defects and threading dislocations. In high-output electronic device using III-nitride semiconductor and SiC, for example, a threading dislocation needs to be reduced to 103/cm2 to obtain a sufficient transistor performance in view of suppressing leak current and ensuring withstand voltage. Also in a light emitting device comprised of a III-nitride semiconductor, a defect density in a III-nitride semiconductor film is preferably about 105/cm2 to further improve light-emitting efficiency and lifetime. A presently optimum method is to use a substrate equal or near to a targeted semiconductor film in lattice constant to achieve a lower defect density. A SiC substrate different from GaN in a lattice constant of as small as 3% is available as a substrate for III-nitride semiconductor layer. The SiC substrate is effective as a substrate for high quality SiC film. The SiC substrate, however, is not suited industrially because it is expensive as compared to a sapphire or a Si substrate.
Accordingly such a lower defect density needs to be realized by the use of a sapphire or a Si substrate. It is not enough to use a single effect produced by the above buffer layer or patterning for this purpose. The number of threading dislocations that can be suppressed by the buffer layer only is limited. There always exist finite threading dislocations or defects of 107/cm2 or more on the grown-film surface. Even if a patterning effect is used, it is difficult to control precisely defects, and there still exist a large number of defects that do not disappear. In an example shown in FIG. 3, for instance, a high-defect-density area exists in a part of the surface and, in addition, the asymmetry of opposing facets causes dislocations to extend obliquely relative to the substrate surface after meeting, which may again increase dislocation density on the grown film surface. In an example shown in FIG. 4 as well, since defects are produced at random positions on facets 202, there exist a large number of dislocations and defects that do not meet and reach the surface.
The above description requires use of plural effects, for example, to decrease substantially the threading dislocation and defects. Disclosure in Japanese Patent Application Laid-Open Publication No. 6-216037 is an example of a prior art to ensure high quality crystal by combining the buffer layer with pattering. FIG. 5 shows the example of prior art in the publication. This is so constructed that a semiconductor layer 302 different from a semiconductor substrate 301 in lattice constant is stacked thereon, on which is formed a large number of teeth 303 with (111) plane, a buffer layer 304 for suppressing the propagation of threading dislocations is stacked throughout, and a semiconductor layer 305 comprised of the same material as or different material from the layer 302 is stacked. Defects 306 extending from layer 302 are bent by the buffer layer, extend toward [111] direction along the facets, meet the defects extending from the opposing facets, disappear and decrease. Unlike the example of prior art shown in FIG. 4, this prior example defines paths of defects, increasing a probability that defects meet together. However, a plurality of defects concentrate in the same path, which makes a defect density very large at the meeting position of defects. It is more probable that defects bend to the perpendicular direction with respect to the substrate and extends to the surface as the threading dislocation instead of completely disappearing.
Since strain is produced in the buffer layer to increase energy when defects are bent, not all defects are bent, some of them are propagated upward directly. Therefore, a defect density is still as high as 5×106/cm2 on the grown-layer surface.